The following description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
LiDAR's are becoming important in sensing applications in a variety of fields associate with autonomous navigation. In particular, autonomous vehicles, UAV's and robotics rely on LiDAR to produce imaging information to be used, with or without information from other sensors, to guide the motion of those platforms. Currently, LiDAR systems consisting of an instrument head (where the light source, the light detectors, the transmission/steering optics, and the detection optics are located) and control and processing assemblies used for the kind of applications made above are housed in a single housing, which is then connected to cables and wires for power and data transmission to a computer or similar capability for image analysis. This implies that the entire package must be placed on the platform, at a location with access to the area for which the imaging is required. For example, for applications with advanced driver assisted systems (ADAS), the LiDAR must be placed on top of the car, or in other locations (say, under the front grill) to have access to the road and roadside. This could be difficult to implement, if the desire is to keep the package from interfering with the structure of the body of the car. This is further complicated if the package must be incorporated with active/passive isolation assemblies to reduce the undesirable effects of shock and vibration that interfere with the proper operation of the LiDAR, or reduce its performance.
LiDAR can be accomplished in a variety of ways. In “time of flight” (TOF) LiDAR short pulses of light are emitted and reflected pulses received, with the delay between emission and reception providing a measure of distance between the emitter and the reflecting object. Such TOF systems, however, have a number of disadvantages. For example simple TOF measurements, in relying solely on the intensity of received light, are highly susceptible to interference from extraneous and irrelevant signal sources. This issue becomes more pronounced as the distance between the emitter and the reflecting object increases, as such distance necessarily decreases the strength of the reflected signal. On the other hand, inherent limitations in accurately measuring extremely short time intervals limit the spatial resolution of such TOF LiDAR systems at close range. In addition, the range of such TOF LiDARs is a function of the ability to detect the relatively faint reflected signal. The resulting range limitations are frequently addressed by using highly sensitive photodetectors. In some instances such detectors can detect single photons. Unfortunately this high degree of sensitivity also leads to increased misidentification of interfering signals as reflect TOF LiDAR pulses from objects other than the target or from other light sources. Despite these disadvantages TOF LiDAR systems currently find wide application, primarily due to the ability to provide such systems in a very compact format and the ability to utilize relatively inexpensive non-coherent laser light sources.
One approach to resolving this problem is to provide a ToF LiDAR system in which system components are distributed about the vehicle. An example of such an approach is found in United States Patent Application No. 2017/0153319 (to Villeneuve and Eichenholz). All publications identified herein are incorporated by reference to the same extent as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply. In such an approach some components of a ToF LiDAR system (such as transmitting scanner, input for gathering reflected light, and a receiver for characterizing the reflected light) can be positioned as a unit at one location of a vehicle while another component (such as the light source) can be positioned elsewhere. The distributed components can be connected using conventional fiber optics and electrical cabling. The time-dependent nature of TOF LiDAR, however, requires proximity of the transmitting scanner, input, and receiver in order to retain timing fidelity. As a result the scanning/receiving assembly of such systems remains relatively large and complex.
Alternatives to TOF LiDAR have been developed. One of these, frequency modulated (FM) LiDAR, relies on a coherent laser source to generate repeated waveforms representing a change of frequency with time or “chirps” of time delimited, frequency modulated optical energy. The frequency within waveform or chirp varies over time, and measurement of the phase and frequency of an echoing waveform or chirp relative to a reference signal provides a measure of distance and velocity of the reflecting object relative to the emitter. Other properties of the reflected chirp (for example, intensity) can be related to color, surface texture, or composition of the reflecting surface. In addition, such FM LiDARs are relatively immune to interfering light sources (which tend to produce non-modulated signals that are not coherent with the received signal) and do not require the use of highly sensitive photodetectors.
Unfortunately, FM LiDAR systems that have been developed to date are generally not compact, as they rely on relatively large FMCW laser sources. In addition, such systems typically rely on a carefully modulated, low noise local oscillator (for example, a narrow linewidth solid state, gas, or fiber laser) with frequency modulation corresponding to that of the emitted chirp provided by a relatively large interferometer. This local oscillator precisely replicates an emitted waveform or chirp, and serves as the reference for the received reflected signal. As a result FM LiDARs are typically large, complex, and expensive, and have seen limited implementation relative to TOF LiDARs despite their performance advantages.
Thus, there is still a need for effective and economical LiDAR systems that provide thorough coverage of the area surrounding a motor vehicle.